Numerous applications require controlling a defined environment's air quality characteristics, including providing an environment having a narrow range of relative humidity (RH) and/or preventing the creation of extreme relative humidity conditions. Indoor air quality can be impacted by air contaminants such as humidity, volatile organic compounds (VOC), semi volatile organic compounds (SVOC), and particulate material. Hence, it is desirable to control not only the air quality, but also relative humidity in order to provide a good quality of indoor air at a low electric energy consumption.
Most prior art relative humidity sensors are capacitive sensors that include a thermosetting polymer as a dielectric layer. The thermosetting polymer possesses a high capability of water vapor absorption. The water vapors can be adsorbed and absorbed by the thermosetting polymeric dielectric and the dielectric constant of the polymer can be increased, which can further increase the capacitance value. Such capacitive detection is effective for integrated humidity sensors, where the parasitic capacitances of wire bonding the sensor and its associated electronics are eliminated.
Problems associated with such capacitive sensors includes cost, a lack of long term performance stability and poor drift behavior due to poor baseline stability (i.e., recovery of the sensor signal to the same response level in the absence of the humidity to be detected). Other problems include aging of the sensing material, which may exhibit or contribute to the baseline drift and poor accuracy. Furthermore, prior art differential resonant sensing approaches for measuring different gases and bio-chemicals often include a coated sensing layer and an uncoated reference surface. The coated sensing layer can sense variants, while the uncoated reference layer can be employed to eliminate common disturbances. Such an uncoated reference layer may not be efficient in removing all common mode signals.
Based on the foregoing, it is believed that a need exists for an improved differential resonant sensor apparatus and method for detecting relative humidity in ambient air. A need also exists for fabricating the differential resonant sensor apparatus in association with a coated reference layer for eliminating the effects of baseline drift, as described in greater detail herein.